But even if you pay the $9-$15 for their Gold account, you still can only get on margin the amount of capital you have in the account max ie: if you deposit $5 and have $5 of cash on hand in your robinhood, but own ~$1200 of stocks, you can use < $1200 of "Gold" which is what they call margin money. It is explained quite well here:

https://support.robinhood.com/hc/en-us/articles/213262686-Ro...

Note that I am a robinhood user and do not use gold or any margin investing. I use it for speculation for long term stuff and have the rest in boring index ETFs / funds primarily. I think the entire idea of investing on margin as playing with fire when you lack fireproof gloves.